1. Field of the Invention
The present invention relates to a butterfly valve of the type which performs valve opening and closing without involving slide engagement of a valve stem with a valve seat. A drive mechanism is provided for rotation and reciprocating motion of the valve stem of the butterfly valve.
2. Description of Background Art
A butterfly valve of this type is disclosed in Japanese Patent Publication No. 46-14779. According to FIG. 8 of the drawings, an upper portion 10a of a valve body 10 facing a valve seat 1f is slidably fitted over a tapered portion 50 of a valve stem 11f. Up and down movement of the valve stem 11f causes oscillation of the upper portion 10a of the valve body 10 in the direction shown by F so that the upper portion 10a is brought in pressure engagement with and disengaged from the valve seat 1f. This arrangement has an advantage in that the valve body 10 can be rotated when it is away from the valve seat 10. Thus, it is possible to conveniently avoid wear of the valve body 10. However, this butterfly valve has a disadvantage in that the tapered portion 50 of the valve stem 11f is positioned within a valve casing 51 in which fluid flows. The fluid may be thrust into a contact area between the tapered portion 50 and the valve stem 11f, thus hindering the movement of the elements.
A butterfly valve intended to overcome this disadvantage is disclosed in Japanese Patent Laid-Open Publication No. 58-28061. According to FIG. 9 of the drawings, a valve stem 11 connected to a valve body 10 is formed with a spherical protuberance 25 where the valve stem 11 and the valve body 10 are pivotally and rotatably supported. Oscillation and rotation of the valve stem 11 and valve body 10 is controlled at the outer end side of the valve stem 11.
This arrangement provides advantages in that the valve opening and closing can be carried out without involving a slide engagement of the valve body 10 with a valve seat 1e. In addition, thrusting of the fluid into a contact area between the valve body 10 and the valve stem 11 can be prevented. The force required in the oscillation of the valve body 10 and valve stem 11 about the spherical protuberance 25 is very small because the principles of the lever and the fulcrum are utilized in the connection.
In this type of butterfly valve, consideration is required to prevent the fluid within the valve casing from leaking outwardly from the location at which the valve stem 11 is fitted in position. For this purpose, packing members 52, such as O rings, are mounted in position at the location of the spherical protuberance 25.
However, the use of packing 52 in abutment with the spherical protuberance 25 constitutes a deviation from the primary conditions for the use of packing. This type of packing cannot only provide a good seal when compared with, for example, the packing as illustrated in FIG. 10 in which packing 54 is held in contact with a conventional rotary shaft 53. The packing 52 makes it extremely difficult to provide proper contact of the packing with the spherical surface for satisfactory sealing.
The prior art valve arrangements are very likely to cause fluid leaks from the sealing portion at the valve stem 11. The leaking at the valve stem 11 is a serious problem.